Streamline setup instructions for common case

[curiousleo]

We currently have the top-level README.md and example/README.md and contrib/README.md. My proposal is this:

• have one short set of instructions that should work for most lorri users, with explicit requirements, in the top-level README.md
• provide help for setting up lorri on other systems, with additional files to help the process along, in contrib.

To actually use lorri, the following are required:

• direnv must be installed and enabled
• lorri must be installed and the daemon must be running
• the user must have a project with an .envrc that invokes lorri direnv

The idea here is that for people using either NixOS or home-manager and a Nix channel that is at least as recent as nixos-19.09, this is really easy. I believe that a lot of lorri users meet these requirements, so I think that it would make sense to optimise the "README experience" for those users.

Concretely, this means:

• rewrite sections "Install" and "Usage" of the top-level README.md in a way that guides users who meet the requirements above through the fast path
• point users who don't meet the requirements to additional resources (e.g. the direnv help pages or the systemd contrib files in contrib) in the "Install" and "Usage" sections of the top-level README.md
• take the Emacs section of example/README.md, put it into contrib/ and link to it from the top-level README.md, then remove the remainder of the example/ directory

(At this point, the example/README.md will no longer be necessary since all the information in here should now be contained in the top-level README.me in a streamlined form.)

Related issues: #227, #226, #96

[curiousleo]

With #233 merged, I think this issue can be closed now.